Chemotrophic Bacteria Research Articles

The resting cyst of dinoflagellate Scrippsiella acuminata host bacterial microbiomes with more diverse trophic strategies under conditions typically observed in marine sediments.

Variation in the condition of marine sediments provides selective preservation milieus, which act as a key determinant for the abundance and distribution of dinoflagellate resting cysts in natural sediments. Microbial degradation is an understudied biological factor of potential importance in the processes. However, gaps remain in our knowledge about the fundamental information of the bacterial consortia associated with dinoflagellate resting cysts both in laboratory cultures and in the field. Here we used Scrippsiella acuminata as a representative of cyst-producing dinoflagellates to delineate the diversity and composition of bacterial microbiomes co-existing with the laboratory-cultured resting cysts, and to explore possible impacts of low temperature, darkness, and anoxia (the mock conditions commonly observed in marine sediments) on the associated bacterial consortia. Bacterial microbiome with high diversity were revealed associated with S. acuminata at resting stage. The mock conditions could significantly shift bacterial community structure and exert notably inhibitory effects on growth-promoting bacteria. Resting cysts under conditions typically observed in marine sediments fostered bacterial microbiomes with more diverse trophic strategies, characteristic of prominently enriched anaerobic chemotrophic bacteria generating energy via respiration with several different terminal electron acceptors, which yielded more acidic milieu unfavorable for the preservation of calcareous resting cysts. Our findings suggest that there is complex and dynamic interaction between dinoflagellates resting cysts and the associated bacterial consortia in natural sediments. This intrinsic interaction may influence the maintenance and/or accumulation of dinoflagellate resting cysts with potential of germination and initiation blooms in the field.

Interaction of Low-Density Polyethylene Nanofragments with Autotrophic and Chemotrophic Bacteria

Interaction of Low-Density Polyethylene Nanofragments with Autotrophic and Chemotrophic Bacteria

Bottom-up construction and screening of algae-bacteria consortia for pollutant biodegradation.

Microbial communities have been used as important biological tools for a variety of purposes associated with agriculture, the food industry and human health. Artificial engineering of microbial communities is an emerging field of research motivated by finding stable and efficient microbial systems. However, the successful design of microbial communities with desirable functions not only requires profound understanding of microbial activities, but also needs efficient approaches to piece together the known microbial traits to give rise to more complex systems. This study demonstrates the bottom-up integration of environmentally isolated phototrophic microalgae and chemotrophic bacteria as artificial consortia to bio-degrade selected volatile organic compounds (VOCs). A high throughput screening method based on 96-well plate format was developed for discovering consortia with bioremediation potential. Screened exemplar consortia were verified for VOCs degradation performance, among these, certain robust consortia were estimated to have achieved efficiencies of 95.72% and 92.70% and near 100% removal (7 days) of benzene, toluene, and phenol, respectively, with initial concentrations of 100 mg/L. VOCs degradation by consortia was mainly attributed to certain bacteria including Rhodococcus erythropolis, and Cupriavidus metallidurans, and directly contributed to the growth of microalgae Coelastrella terrestris (R = 0.82, p < 0.001). This work revealed the potential of converting VOCs waste into algal biomass by algae-bacteria consortia constructed through a bottom-up approach. The screening method enables rapid shortlisting of consortia combinatorial scenarios without prior knowledge about the individual strains or the need for interpreting complex microbial interactions.

Transcriptome analysis of gene expression profiling from the deep sea in situ to the laboratory for the cold seep mussel Gigantidas haimaensis

BackgroundThe deep-sea mussel Gigantidas haimaensis is a representative species from the Haima cold seep ecosystem in the South China Sea that establishes endosymbiosis with chemotrophic bacteria. During long-term evolution, G. haimaensis has adapted well to the local environment of cold seeps. Until now, adaptive mechanisms responding to environmental stresses have remained poorly understood.ResultsIn this study, transcriptomic analysis was performed for muscle tissue of G. haimaensis in the in situ environment (MH) and laboratory environment for 0 h (M0), 3 h (M3) and 9 h (M9), and 187,368 transcript sequences and 22,924 annotated differentially expressed genes (DEGs) were generated. Based on Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, these DEGs were enriched with a broad spectrum of biological processes and pathways, including those associated with antioxidants, apoptosis, chaperones, immunity and metabolism. Among these significantly enriched pathways, protein processing in the endoplasmic reticulum and metabolism were the most affected metabolic pathways. These results may imply that G. haimaensis struggles to support the life response to environmental change by changing gene expression profiles.ConclusionThe present study provides a better understanding of the biological responses and survival strategies of the mussel G. haimaensis from deep sea in situ to the laboratory environment.

Hydrogenophilus thiooxidans sp. nov., a moderately thermophilic chemotrophic bacterium unable to grow on hydrogen gas, isolated from hot spring microbial mats.

A novel thermophilic chemotrophic bacterium, strain SS56T, was isolated from Nakabusa Hot Spring, Japan. The isolate was a rod-shaped (1.5-2.1×0.6-0.8 µm), Gram-stain-negative bacterium. The cells of this strain grew chemoheterotrophically under aerobic and anaerobic conditions. Autotrophic growth was observed with thiosulphate and elemental sulphur under aerobic conditions but not with H2 as the electron donor. Heterotrophic growth in the presence of O2 occurred on yeast extract, tryptone, polypeptone and organic acids. Strain SS56T used nitrite as an alternative electron acceptor under anaerobic chemoheterotrophic conditions. The isolate grew between 35 and 65 °C, with the optimum at 55 °C. The pH range for growth was pH 6.0-9.0; optimal growth occurred at pH 7.0-8.0. The 16S rRNA gene sequence of strain SS56T was 98.9% identical to that of Hydrogenophilus thermoluteolus TH-1T. The draft genome sequence of 2401804 bp for strain SS56T gave values of 53.7% for digital DNA-DNA hybridization, 92.9% for average nucleotide identity and 93.6% for average amino acid identity compared with the genome sequence of 2223143 bp for H. thermoluteolus TH-1T. Based on the information described above, strain SS56T (=DSM 111892T=JCM 34254T) is proposed as the type strain of a novel species, Hydrogenophilus thiooxidans sp. nov.

Genomic Comparison, Phylogeny and Taxonomic Reevaluation of the Ectothiorhodospiraceae and Description of Halorhodospiraceae fam. nov. and Halochlorospira gen. nov.

The Ectothiorhodospiraceae family represents purple sulfur bacteria of the Gammaproteobacteria found primarily in alkaline soda lakes of moderate to extremely high salinity. The main microscopically visible characteristic separating them from the Chromatiaceae is the excretion of the intermediate elemental sulfur formed during oxidation of sulfide prior to complete oxidation to sulfate rather than storing it in the periplasm. We present a comparative study of 38 genomes of all species of phototrophic Ectothiorhodospiraceae. We also include a comparison with those chemotrophic bacteria that have been assigned to the family previously and critically reevaluate this assignment. The data demonstrate the separation of Halorhodospira species in a major phylogenetic branch distant from other Ectothiorhodospiraceae and support their separation into a new family, for which the name Halorhodospiraceae fam. nov. is proposed. In addition, the green-colored, bacteriochlorophyll-containing species Halorhodospira halochloris and Halorhodospira abdelmalekii were transferred to the new genus Halochlorospira gen. nov. of this family. The data also enable classification of several so far unclassified isolates and support the separation of Ectothiorhodospira shaposhnikovii and Ect. vacuolata as well as Ect. mobilis and Ect. marismortui as distinct species.

Achieving nitrogen and phosphorus removal at low C/N ratios without aeration through a novel phototrophic process

Conventional wastewater treatment technologies for biological nutrient removal (BNR) are highly dependent on aeration for oxygen supply, which represents a major operational cost of the process. Recently, phototrophic enhanced biological phosphorus removal (photo-EBPR) has been suggested as an alternative system for phosphorus removal, based on a consortium of photosynthetic microorganisms and chemotrophic bacteria, eliminating the need for costly aeration. However, wastewater treatment plants must couple nitrogen and phosphorus removal to achieve discharge limits. For this reason, a new microalgae-bacterial based system for phosphorus and nitrogen removal is proposed in this work. The photo-BNR system studied here consists of a sequencing batch reactor operated with dark anaerobic, light aerobic, dark anoxic and idle periods, to allow both N and P removal. Results of the study show that the photo-BNR system was able to remove 100% of the 40 mg N/L of ammonia fed to the reactor and 94 ± 3% of the total nitrogen (Influent COD:N ratio of 300:40, similar to domestic wastewater). Moreover, an average of 25 ± 9.2 mg P/L was simultaneously removed in the photo-BNR tests, representing the P removal capacity of this system, which exceeds the level of P removal required from typical domestic wastewater. Full ammonia removal was achieved during the light phase, with 67 ± 5% of this ammonia being assimilated by the microbial culture and the remaining 33 ± 5% being converted into nitrate. The assimilated P corresponded to 2.8 ± 0.23 mg P/L, which only represented, approximately, 1/9 of the P removal capacity of the system. Half of the nitrified ammonia was subsequently denitrified during the dark anoxic phase (50 ± 24%). Overall, the photo-BNR system represents the first treatment alternative for N and P from domestic wastewater with no need of mechanical aeration or supplemental carbon addition, representing an alternative low-energy technology of interest.

Insights into the cyanosphere: capturing the respective metabolisms of cyanobacteria and chemotrophic bacteria in natural conditions?

Specific interactions have been highlighted between cyanobacteria and chemotrophic bacteria within the cyanosphere, suggesting that nutrients recycling could be optimized by cyanobacteria/bacteria exchanges. In order to determine the respective metabolic roles of the cyanobacterial and bacterial consortia (microbiome), a day-night metatranscriptomic analysis was performed on Dolichospermum sp. (N2 -fixer) and Microcystis sp. (non N2 -fixer) natural blooms occurring successively within a French peri-urban lake. The taxonomical and functional analysis of the metatranscriptoms have highlighted specific association of bacteria within the cyanosphere, driven by the cyanobacteria identity, without strongly modifying the functional composition of the microbiomes, suggesting functional redundancy within the cyanosphere. Moreover, the functional composition of these active communities was driven by the living mode. During the two successive bloom events, it appeared that NH4 + (newly fixed and/or allochthonous) was preferentially transformed into amino acids for the both the microbiome and the cyanobacteria, while phosphate metabolism was enhanced, suggesting that due to a high cellular growth, P limitation might take place within the cyanosphere consortium.

Identification and characterization of endosymbiosis-related immune genes in deep-sea mussels Gigantidas platifrons.

Deep-sea mussels of the subfamily Bathymodiolinae are common and numerically dominant species widely distributed in cold seeps and hydrothermal vents. During long-time evolution, deep-sea mussels have evolved to be well adapted to the local environment of cold seeps and hydrothermal vents by various ways, especially by establishing endosymbiosis with chemotrophic bacteria. However, biological processes underlying the establishment and maintenance of symbiosis between host mussels and symbionts are largely unclear. In the present study, Gigantidas platifrons genes possibly involved in the symbiosis with methane oxidation symbionts were identified and characterized by Lipopolysaccharide (LPS) pull-down and in situ hybridization. Five immune related proteins including Toll-like receptor 2 (TLR2), integrin, vacuolar sorting protein (VSP), matrix metalloproteinase 1 (MMP1), and leucine-rich repeat (LRR-1) were identified by LPS pull-down assay. These five proteins were all conserved in either molecular sequences or functional domains and known to be key molecules in host immune recognition, phagocytosis, and lysosome-mediated digestion. Furthermore, in situ hybridization of LRR-1, TLR2 and VSP genes was conducted to investigate their expression patterns in gill tissues of G. platifrons. Consequently, LRR-1, TLR2, and VSP genes were found expressed exclusively in the bacteriocytes of G. platifrons. Therefore, it was suggested that TLR2, integrin, VSP, MMP1, and LRR-1 might be crucial molecules in the symbiosis between G. platifrons and methane oxidation bacteria by participating in symbiosis-related immune processes.Electronic supplementary materialSupplementary material (Supplementary Tables S1–S2) is available in the online version of this article at 10.1007/s00343-020-0040-7.

Characterization of a Sequential UV Photolysis-Biodegradation Process for Treatment of Decabrominated Diphenyl Ethers in Sorbent/Water Systems.

Decabrominated diphenyl ether (BDE-209) is a primary component of the brominated flame retardants used in a variety of industrial and domestic applications. BDE-209 bioaccumulates in aquatic organisms and has been identified as an emerging contaminant that threatens human and ecosystem health. Sequential photolysis-microbial biodegradation processes were utilized here to treat BDE-209 in clay- or soil-water slurries. The removal efficiency of BDE-209 in the clay-water slurries was high; i.e., 96.5%, while that in the soil-water slurries was minimal. In the clay-water slurries the first order rate constants for the UV photolysis and biodegradation of BDE-209 were 0.017 1/day and 0.026 1/day, respectively. UV wavelength and intensity strongly influenced the BDE-209 photolysis and the subsequent biodegradation of photolytic products. Facultative chemotrophic bacteria, including Acidovorax spp., Pseudomonas spp., Novosphingobium spp. and Sphingomonas spp., were the dominant members of the bacterial community (about 71%) at the beginning of the biodegradation; many of these organisms have previously been shown to biodegrade BDE-209 and other polybrominated diphenyl ether (PBDE) congeners. The Achromobacter sp. that were isolated (NH-2; NH-4; NH-6) were especially effective during the BDE-209 degradation. These results indicated the effectiveness of the sequential UV photolysis and biodegradation for treating certain BDE-209-contaminated solids; e.g., clays; in bioreactors containing such solids as aqueous slurries. Achieving a similar treatment effectiveness for more heterogeneous solids containing natural organic matter, e.g., surface solids, appears to be significantly more difficult. Further investigations are needed in order to understand the great difference between the clay-water or soil-water slurries.

Performance and mechanism of carbon dioxide fixation by a newly isolated chemoautotrophic strain Paracoccus denitrificans PJ-1

CO2 is regarded as a major contributor to the global warming. CO2 utilization is promising to reduce the CO2 emissions. Currently, the biofixation of CO2 using chemoautotrophs has markedly gain interest in CO2 utilization. In this study, a newly isolated chemoautotroph, Paracoccus denitrificans PJ-1, was used for the biofixation of CO2 under anaerobic condition. Experimental results revealed that Paracoccus denitrificans PJ-1 achieved a high carbon fixation rate (13.25 mg·L−1·h−1) which was ∼10 times faster than the previous reported chemotrophic bacteria using thiosulfate as electron donor. The best CO2 fixation activity of Paracoccus denitrificans PJ-1 was achieved at the pH value of 9.0 and CO2 concentration of 20 vol%. Meanwhile, a high CO2 fixation yield of 106.03 mg·L−1 was reached. The presence of oxygen was adverse to the biofixation, indicating that strain PJ-1 was more suitable for CO2 fixation in anaerobic environments. Carbon mass balance analysis revealed that the carbon from CO2 was mainly fixed into the extracellular organic carbon rather than the biomass. GC-MS analysis and cbbL gene test revealed that Paracoccus denitrificans PJ-1 fixed CO2 through the Calvin-Benson-Bassham cycle and mainly converted CO2 to oxalic acid and succinic acid. Overall, the excellent CO2 fixation capacity of Paracoccus denitrificans PJ-1 suggests that it had potential for CO2 utilization.

Resource partitioning among brachiopods and bivalves at ancient hydrocarbon seeps: A hypothesis.

Brachiopods were thought to have dominated deep-sea hydrothermal vents and hydrocarbon seeps for most of the Paleozoic and Mesozoic, and were believed to have been outcompeted and replaced by chemosymbiotic bivalves during the Late Cretaceous. But recent findings of bivalve-rich seep deposits of Paleozoic and Mesozoic age have questioned this paradigm. By tabulating the generic diversity of the dominant brachiopod and bivalve clades-dimerelloid brachiopods and chemosymbiotic bivalves-from hydrocarbon seeps through the Phanerozoic, we show that their evolutionary trajectories are largely unrelated to one another, indicating that they have not been competing for the same resources. We hypothesize that the dimerelloid brachiopods generally preferred seeps with abundant hydrocarbons in the bottom waters above the seep, such as oil seeps or methane seeps with diffusive seepage, whereas seeps with strong, advective fluid flow and hence abundant hydrogen sulfide were less favorable for them. At methane seeps typified by diffusive seepage and oil seeps, oxidation of hydrocarbons in the bottom water by chemotrophic bacteria enhances the growth of bacterioplankton, on which the brachiopods could have filter fed. Whereas chemosymbiotic bivalves mostly relied on sulfide-oxidizing symbionts for nutrition, for the brachiopods aerobic bacterial oxidation of methane and other hydrocarbons played a more prominent role. The availability of geofuels (i.e. the reduced chemical compounds used in chemosynthesis such as hydrogen sulfide, methane, and other hydrocarbons) at seeps is mostly governed by fluid flow rates, geological setting, and marine sulfate concentrations. Thus rather than competition, we suggest that geofuel type and availability controlled the distribution of brachiopods and bivalves at hydrocarbon seeps through the Phanerozoic.

Microbial Ecology of Biofiltration Units Used for the Desulfurization of Biogas

Bacterial communities’ composition, activity and robustness determines the effectiveness of biofiltration units for the desulfurization of biogas. It is therefore important to get a better understanding of the bacterial communities that coexist in biofiltration units under different operational conditions for the removal of H2S, the main reduced sulfur compound to eliminate in biogas. This review presents the main characteristics of sulfur-oxidizing chemotrophic bacteria that are the base of the biological transformation of H2S to innocuous products in biofilters. A survey of the existing biofiltration technologies in relation to H2S elimination is then presented followed by a review of the microbial ecology studies performed to date on biotrickling filter units for the treatment of H2S in biogas under aerobic and anoxic conditions.

Cambrian Photosynthetic and Chemosynthetic Ecosystems in the Siberian Craton

The leading role of bacterial communities in the formation of marine sedimentary rocks is inferred from the synthesis of expert knowledge about bacterial paleontology, while the results of oceanic studies (materials and data) offer new insights into symbiotic relationships between phototrophic and chemotrophic bacteria and fauna.

The stable isotope fingerprint of chemosymbiosis in the shell organic matrix of seep-dwelling bivalves

Chemosymbiotic bivalves harboring endosymbiotic, chemotrophic bacteria have been investigated from a variety of hydrocarbon seeps worldwide. It has been shown that carbon, nitrogen, and sulfur isotopic compositions of the animal soft body parts are excellent indicators for evaluating energy transfer and food sources for the respective deep-sea habitats. However, recognition of chemosymbiosis has proven to be difficult for bivalves that dwelled at ancient seeps due to the lack of soft tissue. Here, we investigated δ13C, δ15N, and δ34S signatures of the tissue (mantle) and the shell organic matrix (SOM) of the same specimens of three bathymodiolin mussel species with different chemotrophic symbionts (methanotrophs in Bathymodiolus platifrons and B. childressi and thiotrophs in B. aduloides) and one vesicomyid clam (Calyptogena sp.) from a variety of hydrocarbon seeps from the South China Sea and the Gulf of Mexico. The data obtained demonstrate that all seep bivalves regardless of species or locations reveal overall small differences in δ13C (≤+4‰), δ15N (≤+1‰), and δ34S (≤+5‰) values between SOM and mantle (∆SOM-mantle) of the specimens. Relatively larger ∆SOM-mantle for δ13C values (as high as +10‰) in B. platifrons and larger ∆SOM-mantle for δ34S values (up to 16‰) in B. aduloides and Calyptogena sp. might be due to different symbionts in their gills. Since SOM can be extracted from fossil bivalve shells, the proxy can be used as a fingerprint of chemosynthesis-based food chains, although its utility will depend on the quality of preservation of the shell organic matter. Despite this uncertainty, the new proxy has great potential to reconstruct energy flow through different types of chemosynthesis-based ecosystems.

Light Modulates Metabolic Pathways and Other Novel Physiological Traits in the Human Pathogen Acinetobacter baumannii

Light sensing in chemotrophic bacteria has been relatively recently ascertained. In the human pathogen Acinetobacter baumannii, light modulates motility, biofilm formation, and virulence through the blue-light-sensing-using flavin (BLUF) photoreceptor BlsA. In addition, light can induce a reduction in susceptibility to certain antibiotics, such as minocycline and tigecycline, in a photoreceptor-independent manner. In this work, we identified new traits whose expression levels are modulated by light in this pathogen, which comprise not only important determinants related to pathogenicity and antibiotic resistance but also metabolic pathways, which represents a novel concept for chemotrophic bacteria. Indeed, the phenylacetic acid catabolic pathway and trehalose biosynthesis were modulated by light, responses that completely depend on BlsA. We further show that tolerance to some antibiotics and modulation of antioxidant enzyme levels are also influenced by light, likely contributing to bacterial persistence in adverse environments. Also, we present evidence indicating that surfactant production is modulated by light. Finally, the expression of whole pathways and gene clusters, such as genes involved in lipid metabolism and genes encoding components of the type VI secretion system, as well as efflux pumps related to antibiotic resistance, was differentially induced by light. Overall, our results indicate that light modulates global features of the A. baumannii lifestyle.IMPORTANCE The discovery that nonphototrophic bacteria respond to light constituted a novel concept in microbiology. In this context, we demonstrated that light could modulate aspects related to bacterial virulence, persistence, and resistance to antibiotics in the human pathogen Acinetobacter baumannii In this work, we present the novel finding that light directly regulates metabolism in this chemotrophic bacterium. Insights into the mechanism show the involvement of the photoreceptor BlsA. In addition, tolerance to antibiotics and catalase levels are also influenced by light, likely contributing to bacterial persistence in adverse environments, as is the expression of the type VI secretion system and efflux pumps. Overall, a profound influence of light on the lifestyle of A. baumannii is suggested to occur.

Transition from exponential to linear photoautotrophic growth changes the physiology of Synechocystis sp. PCC 6803

Phototrophic microorganisms like cyanobacteria show growth curves in batch culture that differ from the corresponding growth curves of chemotrophic bacteria. Instead of the usual three phases, i.e., lag-, log-, and stationary phase, phototrophs display four distinct phases. The extra growth phase is a phase of linear, light-limited growth that follows the exponential phase and is often ignored as being different. Results of this study demonstrate marked growth phase-dependent alterations in the photophysiology of the cyanobacterium Synechocystis sp. PCC 6803 between cells harvested from the exponential and the linear growth phase. Notable differences are a gradual shift in the energy transfer of the light-harvesting phycobilisomes to the photosystems and a distinct change in the redox state of the plastoquinone pool. These differences will likely affect the result of physiological studies and the efficiency of product formation of Synechocystis in biotechnological applications. Our study also demonstrates that the length of the period of exponential growth can be extended by a gradually stronger incident light intensity that matches the increase of the cell density of the culture.

Chemotrophic bacteria and material flows: At a crossroad between biogeochemistry and ecology

Chemotrophic bacteria and material flows: At a crossroad between biogeochemistry and ecology

Mercury accumulation in hydrothermal vent mollusks from the southern Tonga Arc, southwestern Pacific Ocean

We provide the mercury (Hg) and monomethylmercury (MMHg) levels of the plume water, sulfide ore, sediment, and mollusks located at the hydrothermal vent fields of the southern Tonga Arc, where active volcanism and intense seismic activity occur frequently. Our objectives were: (1) to address the potential release of Hg from hydrothermal fluids and (2) to examine the distribution of Hg and MMHg levels in hydrothermal mollusks (mussels and snails) harboring chemotrophic bacteria. While high concentrations of Hg in the sediment and Hg, As, and Sb in the sulfide ore indicates that their source is likely hydrothermal fluids, the MMHg concentration in the sediment was orders of magnitude lower than the Hg (<0.001%). It suggests that Hg methylation may have not been favorable in the vent field sediment. In addition, Hg concentrations in the mollusks were much higher (10–100 times) than in other hydrothermal vent environments, indicating that organisms located at the Tonga Arc are exposed to exceedingly high Hg levels. While Hg concentration was higher in the gills and digestive glands than in the mantles and residues of snails and mussels, the MMHg concentrations in the gills and digestive glands were orders of magnitude lower (0.004–0.04%) than Hg concentrations. In summary, our results suggest that the release of Hg from the hydrothermal vent fields of the Tonga Arc and subsequent bioaccumulation are substantial, but not for MMHg.

Competition and facilitation between the marine nitrogen-fixing cyanobacterium Cyanothece and its associated bacterial community.

N2-fixing cyanobacteria represent a major source of new nitrogen and carbon for marine microbial communities, but little is known about their ecological interactions with associated microbiota. In this study we investigated the interactions between the unicellular N2-fixing cyanobacterium Cyanothece sp. Miami BG043511 and its associated free-living chemotrophic bacteria at different concentrations of nitrate and dissolved organic carbon and different temperatures. High temperature strongly stimulated the growth of Cyanothece, but had less effect on the growth and community composition of the chemotrophic bacteria. Conversely, nitrate and carbon addition did not significantly increase the abundance of Cyanothece, but strongly affected the abundance and species composition of the associated chemotrophic bacteria. In nitrate-free medium the associated bacterial community was co-dominated by the putative diazotroph Mesorhizobium and the putative aerobic anoxygenic phototroph Erythrobacter and after addition of organic carbon also by the Flavobacterium Muricauda. Addition of nitrate shifted the composition toward co-dominance by Erythrobacter and the Gammaproteobacterium Marinobacter. Our results indicate that Cyanothece modified the species composition of its associated bacteria through a combination of competition and facilitation. Furthermore, within the bacterial community, niche differentiation appeared to play an important role, contributing to the coexistence of a variety of different functional groups. An important implication of these findings is that changes in nitrogen and carbon availability due to, e.g., eutrophication and climate change are likely to have a major impact on the species composition of the bacterial community associated with N2-fixing cyanobacteria.